1. Field of the Invention
This invention relates to apparatus and process for drilling deep holes through hard natural formations, such as rock. More particularly, this invention relates to apparatus and process for cutting rock by means of high-velocity fluid jets containing abrasive particulates. The apparatus and process of this invention relates in one embodiment particularly to a rotating drill bit, through which high-velocity abrasive fluid jets are emitted, that could be lowered into the ground and advances automatically to drill a hole through rock. The apparatus and process of this invention relates in another embodiment to use of two conduits for separately transporting high pressure fluid and abrasive particulate containing slurry or foam over a long distance to a rotary drill bit for generating abrasive fluid jets capable of cutting hard rock. The apparatus and process of this invention are particuarly well suited for drilling wells to extract water, hydrocarbons, minerals, and geothermal resources.
2. Description of the Prior Art
The conventional methods of drilling holes through rock involve the use of a rotating drill bit having cutting edges capable of crushing rock when force is applied by the rotating drill bit against the rock. To provide thrust and rotate the drill bit against rock requires power input and suitable means of transmitting the power from a prime mover to the rotary drill bit. In drilling wells for extracting hydrocarbons, such as oil and natural gas, drill pipes are used to transmit the torque from an engine to a downhole drill bit. This drilling method has many shortcomings that contribute to the high cost of such operations. Conventionally, drill pipes are supplied in 40 foot sections and must be threaded together to form a long distance torque transmitting drill pipe. Threading drill pipes takes time and labor and thus slows down the drilling and increases the cost. Drill pipes are heavy and require the erection of an elaborate tower structure to conduct the drilling operation. The torque transmitting capability of drill pipes is quite limited, thus limiting the level of power that could be applied to the bit. The drill pipe can be jammed against or rub against the rock surface at the side of the hole causing damage to the drill pipe or loss of useful power. Due to the very high level of thrust and torque that must be applied to the cutting edges of the drill bit to cut hard rock, the life of cutting edges is short and frequent replacement of cutting edges results in very costly downtime. In practice, the inability of a drill pipe to transmit high torque and the inability of the drill bit to withstand high thrust and torque limit the power that can be applied to the rock surface to about 10 to 25 hp in conventional rotary drilling.
To overcome the power transmitting limitation of the drill pipe and to prolong the life of a drill bit by reducing the required thrust, various means of high pressure fluid jetting have been attempted. A review of such techniques is set forth in "Novel Drilling Techniques" by W. C. Maurer, Pergamon Press, Oxford, England (1968). In one approach, high pressure fluid is transmitted to the drill bit by means of pipes or high pressure tubing to generate high velocity fluid jets which are directed against the rock surface to cut a groove and to facilitate the cutting edges in breaking the rock. The use of such fluid jets has been shown to reduce the thrust required to cut rock, thus reducing the wear of the drill bit. However, the effectiveness of the fluid jets in cutting grooves is governed essentially by the pressure differential of the high pressure fluid and the down hole ambient fluid pressure and very high fluid pressure, such as that capable of providing a pressure differential of more than 15,000 psi, is required to effectively cut hard rock. In such cases, the transportation of the pressurized fluid from the surface pump to the drill bit becomes a problem if the depth of drilling is substantial. Further, pumping a large volume of fluid to a pressure level such as 30,000 psi also presents a problem. The use of high pressure fluid jets to aid rotary drill bits is discussed in Maurer, W. C. and Heilhecker, J. K., "Hydraulic Drilling", Proceedings, Fourth Symposium on Drilling and Rock Mechanics, University of Texas, Austin (1968), and Maurer, W. C., Heilhecker, J. K. and Love, W. W., "High Pressure Drilling", Journal of Petroleum Technology (July, 1973).
Because of the difficulties in compressing fluid to very high pressures and in transporting the pressurized fluid over a long distance, the fluid jet approach of drilling wells is not currently practiced. However, the benefits of fluid jet augmented rock cutting techniques have been well demonstrated. This approach, in fact, is currently being employed to cut rock in tunneling applications in which high pressure water jets of 40,000 psi or higher are used to cut concentric grooves in rock and trailing disk cutters are employed to crush the concentric rings of rock delineated by the water jet made grooves. This approach resulted in the reduced thrust requirement for equal or better cutting rate such that the power requirement and/or the weight of the tunnel machine could be reduced.
In order to achieve the benefits of the fluid jet assisted drilling, without having to pressurize the fluid to an impractical level, addition of abrasive particules to the fluid to improve the fluid jet's capability in cutting hard rock has been investigated. Selected abrasive particulates have been added to the drilling fluid, which was pumped down hole to a drill bit equipped with suitable jet generating nozzles. The abrasive fluid jet approach of cutting rock in drilling applications has been discussed in the following references: Fair, J. C., "Development of High Pressure Abrasive Jet Drilling", Journal of Petroleum Technology, August, 1981, and U.S. Pat. No. 3,112,800 teaching use of abrasive fluid jet to cut the periphery of a circular rock core, which is subsequently removed by a roller bit; U.S. Pat. No. 3,389,759 teaching placing an abrasive fluid jet nozzle inside a nozzle holder that could be retrieved with a wire line through the drill pipe; U.S. Pat. No. 3,548,960, teaching a rock drill that utilizes abrasive fluid jets to cut kerfs and stand-off elements to mechanically aid in removing the ridges formed between the kerfs; U.S. Pat. No. 3,231,031, teaching an abrasive fluid jet to drill a pilot hole in rock just ahead of a rock bit and a water hammer effect produced by lowering the drillstring to the pilot hole to shut off the fluid flow to impact the bit against the rock and to fracture the rock around the pilot hole; and U.S. Pat. No. 3,324,957, teaching use of high velocity hydraulic jets to drill a pilot hole which is enlarged by high velocity hydraulic jets from a tapered drilling tool. Wyllie, M. R. J., "Jetted Particle Drilling", Proceedings, 8th World Petroleum Congress, Moscow, U.S.S.R. (1971) summarizes extensive tests to impact and abrade selected rock specimens with drilling fluid formulas that contain various types of abrasive materials.
The prior art teachings relative to cutting rock with abrasive containing fluid jets all involve adding selected abrasive materials to the drilling fluid or mud and compressing this mixture to the desired pressure and through the drill pipe to a rotary bit having nozzles capable of forming fluid jets. The abrasive materials include sand and steel shots of selected grain sizes. The prior art demonstrates that with an abrasive containing fluid jet the drill rate against hard rock can be significantly increased and the life of bit prolonged. However, the wear of pump parts, swivel packings, and nozzles by the abrasive materials added to the drilling fluid was found to be excessive and difficult to avoid. As a result, none of these prior art teachings are currently practiced in drilling wells.
There have been various attempts to provide abrasive containing fluid jet streams. U.S. Pat. Nos. 3,424,386, 3,972,150, 4,080,762 and 4,125,969 all teach the abrasive (sand) stream to be in the central portion of the nozzle while the pressurized fluid is introduced into the peripheral area surrounding the central sand stream. A ring orifice plate or disk such as employed in the U.S. Pat. Nos. 3,424,386, 4,080,762 and 4,125,969 to provide the fluid jets around the sand stream has many disadvantages including: the introduction of pressurized fluid tangentially into a nozzle a short distance above the orifice disk is not conducive to the generation of a coherent fluid jet due to flow disturbances upstream of the orifices; sand in the central portion of a nozzle creates an abrasive environment that can weaken the interior wall of the annular fluid chamber without being detected; pressurized fluid in the outer annular space results in a nozzle that is very large in dimensions as both interior and exterior walls must be sized to accommodate the fluid pressure; and sealing the annular orifice disk can be very troublesome. U.S. Pat. No. 3,994,097 teaches a centrally located water jet while sand is fed into a nozzle chamber through a single sand passageway. The sand is forced into the water jet by passage through a conical nozzle. This patent recognizes abrasion problems within the nozzle and the necessity of exact alignment. These problems would be intensified at higher pressures. All of these patents teach mixing abrasive into water by (1) intercepting an abrasive stream with water jets, and (2) forcing abrasives, water and air through a conical nozzle, without concern of fluid actions.
The prior art devices have generally utilized compressed air to deliver the abrasive particles to a nozzle in which the particles are mixed with the water stream. It is desirable, however, for the particles to be wetted by water before they are to be most effectively mixed with the water. Further, if the water stream is coherent and is traveling at high speed, the conditions are not favorable for the air propelled particles to be mixed into the water stream. At best, some particles are carried away by the water droplets formed around the coherent core of the water stream. The introduction of abrasive particles would be significantly improved if the water jet is made to disperse into droplet form, however, the resultant abrasive water jet would be weak and incapable of cutting hard materials.
The transporting of abrasive particles by compressed air or gas also has other undesirable characteristics. Since abrasive particles are generally heavy, the air flow must be sufficiently turbulent to move the particles, otherwise the particles will settle and block the passage. The air or gas must be dry to avoid agglomeration of particles and resulting blockage of the passage. Further, erosion of tubing, hoses and fittings by the abrasive particles is a common problem.
A possible alternative approach of transporting abrasive particles to the nozzle is to convert the abrasives to a slurry as taught by U.S. Pat. No. 3,972,150 relating to a hand held gun. This abrasive slurry is then pumped into a nozzle and mixed with the water jet. One problem of this approach is that the slurry must be mixed into the water jet, the mixing of which can consume a significant amount of the water jet's kinetic energy as the slurry, rather than the individual abrasive particles, must be accelerated to the water jet velocity. Such loss of water jet energy can be particularly severe if the abrasive slurry is viscous. These problems are increased by the fact that high viscosity may be necessary in formulating such an abrasive slurry, if settlement of the particles is to be avoided.